In order to be able to open a window the actuator must be capable of pushing the sash under compression rather than pull it under tension the chain links must be constructed to provide substantial rigidity for the chain when it is in a straight configuration, so that the chain can transmit force to the swinging window part for opening and closing thereof. The chain must also possess the requisite degree of lateral flexure to conform to the arcuate path traversed by swinging sash from the window frame.
U.S. Pat. No. 4,941,316 discloses a push-pull chain for use in a window or door actuator. This known chain comprises a series of links hinged together by pins, the links being provided with faces or lugs which abut in a manner to allow the chain to bending by rotation of the links about their hinges beyond a straight line in one direction only in a very limited extend, but which allow the chain to bend freely in the opposite direction.
Such a chain can transmit compression when the actuator is provided with suitable guides at one or both of its two ends, which guides urge the ends each over a length of one or more links into a configuration in which the chain is bended beyond a straight line with the lugs abutting with one another.
A chain of this known type may thus be employed to transmit both tension and compression round bends, by passing it over a suitable sprocket, it being understood that the bend round the pulley must be in the direction in which the chain is free to bend. A guide sheave, concentric with the pulley must be provided to prevent the chain being pushed off the sprocket when transmitting a compression load.